Abstract

The role of exchange interactions and thermal relaxation in advanced three-layer exchange-coupled composite perpendicular recording media with graded anisotropy was investigated through studies on magnetic viscosity,magnetic hysteresisreptation, and spin-stand adjacent track interference. For this purpose, thickness series in the NiW seed and the three magnetic layers were made and studied. For each sample, both magnetic viscosity and reptation were evaluated using a magnetometer over a series of initial magnetic states. Magnetic viscosity, which is the change in the magnetization of the sample with time, showed a distinct second-order dependence on a logarithmic time-scale. In general, viscosity appeared to be more strongly influenced by exchange-coupling interactions in the media than the thermal stability. Magnetic reptation, which is the change in the magnetization when the external field was repeatedly cycled between a nonzero field and zero field, i.e., remanence, showed two types of distinct reptation phenomena—field reptation and remanence reptation—depending on whether the magnetic state was evaluated with the external field present or removed, respectively. Both field and remanencereptation showed a second-order logarithmic dependence on the number of cycles. A comparison of viscosity and field reptation revealed a correlation, indicating that the origin of field reptation in perpendicular recording media can be associated with viscosity. Such a correlation could not be established between viscosity and remanencereptation. Spin-stand signal amplitude change of written tracks, due to adjacent track interference, also followed a second-order logarithmic dependence on the number of adjacent writes. When compared appropriately, the signal amplitude change showed an excellent correlation with remanencereptation across all media samples. This suggests that adjacent track interference in magnetic recording can be associated with remanencereptation evaluated using a magnetometer, despite their vast difference in time-scales. Moreover, this correlation was observed irrespective of the thermal stability of the media, which corroborates the hypothesis that in composite media, exchange interaction effects have a more dominant role than thermal relaxation effects in influencing switchability vis-à-vis adjacent track interference. In consequence, remanencereptation can be a useful technique to study the interplay of exchange and thermal effects in magnetic media.

Received 15 March 2012Accepted 20 July 2012Published online 28 August 2012

Acknowledgments:

The authors would like to thank L. Bitman and S. Dang for help with the viscosity and reptation measurements, and M. Marino for help with the spin-stand recording measurements. The authors would also like to thank Dr. G. Bertero for a critical reading of the manuscript and useful discussions.